1. Field
The present field relates to a process for producing a polycrystalline bulk semiconductor, and particularly relates to a process for producing polycrystalline bulk Si, Ge, or SiGe.
2. Description of the Related Technology
The casting method is known as a leading process for growing bulk crystals used in low-cost practical solar batteries. The casting method is used to produce a polycrystalline material by nuclei growth from the periphery of a cast, and to mainly grow ingots composed of polycrystalline bulk Si.
An advantage of the casting method is that large quantities of polycrystalline bulk Si can be grown relatively easily at low cost. However, the conventional casting method has a problem in that bulk crystals with good crystal quality are difficult to obtain. For example, when polycrystalline silicon thus obtained is used as a wafer of a solar battery, positive holes and charges generated by photons recombine and conversion inefficiency is reduced when a large amount of grain boundaries or the like is present in the polycrystals due to the fact that the particle size of the bulk polycrystals is small. Since the orientation of the crystal grains is not aligned in the manner of a single crystal, a textured structure cannot be formed, and conversion efficiency is reduced due to surface reflection, when the surface is chemically etched.
For this reason, it is important to improve the quality of the silicon ingot, which is the raw material of the wafer. When polycrystalline silicon is produced using the casting method, a technique is commonly used in which cooling is carried out from the bottom portion of the crucible, and crystal growth is accelerated toward the upper portions by controlling the temperature in the vertical direction. However, even when polycrystalline silicon is solidified in a single direction by using such temperature control, crystals grow from a large amount of growth nuclei in the bottom portion of the crucible. The result is that because the crystal grains are large and have nonuniform dimensions, the grain sizes are nonuniform, a large number of grain boundaries and the like is formed, and good polycrystalline silicon cannot be obtained.
To improve crystallinity in the casting method, it has been proposed that seed crystals be disposed on the bottom portion of the crucible that holds molten silicon, and good-quality single crystals or bulk polycrystals be obtained by growing crystals from the seed crystals. However, even if a technique is used in which seed crystals are disposed over the entire surface of the bottom portion of the crucible, the technique is still disadvantageous in terms of controllability and cost, residual strain increases because the growth of crystal grains is naturally inhibited, and other unwanted effects occur, making it impossible to achieve a result in which starting materials melted inside the crucible can grow while retaining the good crystallinity of the seed crystals.
It has also been proposed that solidification and growth be accelerated by generating dendrites in the initial stage of solidification of the casting method in order to improve crystallinity (see Patent Document 1).
Dendrites are dendritic crystals and the crystals are observed to be dendritic because the growth velocity in a specific direction of the crystal is rapid. Conventionally, dendrites are not observed to grow along the bottom surface of the crucible during the crystal growth of silicon.
Patent document 1 describes growing dendrites in the initial stage of solidification, and the orientation in the growth direction to be a {111} plane. However, a {111} plane is a stable growth plane of polycrystalline silicon, and is a plane that is naturally manifest in ordinary casting methods without the use of dendrite crystals. Therefore, one of skill in the art understands that the process disclosed in Patent Document 1 is no different in any manner from an ordinary casting method. In fact, the plane that appears is only a {112} plane or a {110} plane in the case that dendrite crystals are grown along the bottom surface of a crucible and it is essentially impossible to align the orientation of the crystal grains in a {111} plane using dendrite crystals (see diagram on the left in FIG. 11). Specifically, one of skill in the art generally understands that all crystal grain orientations in a {111} plane cannot be aligned by casting, no matter what technique is used.
In other words, only the natural orientation ratio (ordinarily 50% to 60%) can be achieved in a {111} plane, and this is a result that has already been achieved by ordinary methods.
The process disclosed in Patent Document 1 is not a process that is designed to control the growth of dendrites in a perpendicular direction (the direction along the bottom surface) with respect to the direction of growth of the bulk crystal. Therefore, it is difficult to match the crystal grain size and the crystal grain direction in the growth direction.
Patent Document 1. Japanese Laid-open Patent Application No. 2005-132671